Switch-mode power amplifiers have attracted a significant amount of interest for use in applications requiring highly efficient power amplification of high frequency signals. Examples of applications of such devices include power amplifiers for wireless communications systems, satellite communications systems, and advanced radar systems. In particular, high power, high frequency power amplifiers are needed for digital communication systems such as 3G and 4G PCS systems, WiFi, WiMax and digital video broadcast systems. For applications requiring high output power, the power amplifier accounts for a significant portion of the overall power consumed by the system. Thus, it is desirable to maximize the efficiency of the power amplifier circuit in a communication system.
Switch-mode amplifiers, for example amplifiers operating in Class D, E, F or S, provide high efficiency amplification by minimizing the amount of power dissipated in the transistor. Since switch-mode amplifiers are highly non-linear, they are particularly suitable for use in connection with digital systems which employ constant-envelope modulation schemes. However, switch-mode amplifiers can also be used in other applications by providing additional circuitry for extracting amplitude information from the input signal and restoring the amplitude information to the output signal.
In a Class E amplifier, the transistor operates as an on-off switch whose state is driven by a time-varying input signal. The binary output of the transistor is applied to a reactive load network which filters out harmonic components of the transistor output signal, resulting in a narrow bandwidth, amplified output signal. In the Class E configuration, the transistor drain current is minimized (ideally zero) whenever a drain voltage is present on the device, and the drain voltage is minimized (ideally zero) whenever there is a drain current passing through the device. Since the power dissipated in the transistor is equal to the instantaneous product of the drain current and the drain voltage averaged over a period, the power loss in a Class E device is ideally zero. Thus, a Class E device can theoretically operate at 100% drain efficiency. The efficiency of actual devices is lower than 100%. Nevertheless, very high efficiencies can be realized in Class E amplifiers.
To date, Class E amplifiers have been realized primarily using narrow bandgap (silicon and GaAs) bipolar, MOSFET and MESFET technology. Bipolar and MOSFET devices have demonstrated high power output at frequencies below 1.0 GHz. However, such devices are unsuitable for higher frequency applications which demand higher transistor switching speeds. High frequency amplifiers (i.e. amplifiers capable of operating at frequencies at or above 1.0 GHz) have been realized using GaAs MESFETs. However, the resulting amplifiers are not capable of high power output desired for communications applications (particularly base station applications), and the efficiencies of such devices are less than ideal. In particular, GaAs MESFET devices have limited power density and limited drain voltage, which limits the amount of power they can produce.
Accordingly, there is a continued need for a single-stage switch-mode amplifier circuit capable of producing in excess of 10 W of output power at frequencies exceeding 1.0 GHz. Furthermore, there is a need for a switch-mode amplifier circuit capable of power added efficiency in excess of 75% at frequencies exceeding 1.0 GHz.